In the realm of battery powered, portable consumer electronic devices such as smart phones, laptop computers and tablet computers, there are a number of sensitive electronic sub-systems that operate relatively tight timing margins, based on a clock signal. Examples of such sub-systems are an applications processor (or system-on-a-chip), a cellular phone network communications interface, a digital camera, a touchscreen, and various sensors such as a proximity sensor and an inertial sensor. Furthermore, such sub-systems often need to be located very close to each other, due to the limited space that is available within the outer enclosure or package of such devices. As a result, it is desirable to reduce electromagnetic noise or interference (EMI) that could be due to the magnetic fields generated by the inductors of switch mode power converters. Otherwise, EMI could present a problem when the fundamental or operating clock frequency of a subsystem is very close to (or aligned with) the switching frequency of the power transistors of a nearby power converter.
A switch mode power converter, such as dc-dc voltage regulator, is sometimes operated in Pulse Frequency Modulation (PFM) mode (i.e., not a fixed frequency pulse width modulation (PWM) mode) in order to improve its efficiency under light load conditions. Typical implementations include, Burst Mode, Pulse Skip Mode etc. where the switching frequency varies with the load current and other operating parameters while at the same time maintaining a regulated DC output voltage. The switch mode controller varies the switching frequency as a function of the load (as well as other parameters) while at the same time maintaining a regulated DC output voltage. For a load that can range for example anywhere from zero to several hundred milliAmps, the converter exhibits a very broad range of power transistor switching frequency, for example from 0 Hz into the MHz range, and also varying inductor magnetic field ripple. Thus, potentially interfering signals are generated over a very wide frequency range.
One possible solution to the problem of EMI generation and susceptibility within portable devices is to add EMI shielding between the power converter circuitry and nearby sensitive circuits, or to at least position the inductors of the power converter away from the sensitive circuits. That however may not be practical within space-constrained, portable devices especially tablet computers and smartphones. Other solutions to the problem may lie in the use of spread spectrum techniques where the total switching noise power is spread across a wider frequency range (by for example randomizing a switching parameter of the power converter), so that noise power at the operating frequency of a nearby subsystem is reduced. The trade off in the spread spectrum approach however is that the noise floor is raised.